D-Link DFL-2500 User Guide - Page 94
Area Border RouterABR, Autonomous System Boundary RoutersASBRs, stub area, Designated RouterDR,
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10.3. Routing Algorithms 75 configured to connect to the backbone through a non-backbone area. VLink can also be used to link through partitioned backbone areas. A normal OSPF area acts like a private network connecting to the backbone area via some router called Area Border Router(ABR). ABRs have interfaces in more than one area, and maintains separate routing information databases for each area to which they are connecting by an interface. The routers reside in the same OSPF area only need to learn and synchronize link-state information with the ABR. Some Routers that exchange routing information with routers in other ASs are called Autonomous System Boundary Routers(ASBRs). ASBRs introduce externally learned routes to the AS and flood the external routing advertisement throughout all OSPF normal areas. To reduce the flooding traffic of external routes advertisement, a special area called "stub area" can be configured. When a stub area is configured, the ABR will automatically advertise a default route so that routers in the stub area can reach destinations outside the area. The default route becomes the single exit point of the stub area, and the area will not accept broadcasts of external routes. Operating Process Establishment - "Hello" At the initialization stage, each router within an area detects its directly connected network, and sends "Hello" packets to all its OSPF enabled interfaces to determine who their neighboring routers are. Routers having interfaces directly connected and residing in the same OSPF area become neighbors. When a router sends and receives "Hello" packets and detects multiple routers in an AS, it will select a Designated Router(DR) and also a Backup Designated Router(BDR) for further link-state information exchange. DR and BDR are automatically elected by "Hello" protocol on every OSPF broadcast network. The Router Priority which is configurable on a per-interface basis is the parameter that controls the election. The router with the highest priority number becomes D-Link Firewalls User's Guide